Proper irrigation management requires that growers assess
their irrigation needs by taking measurements of various physical parameters.
Some use sophisticated equipment while others use the tried and true
common sense approaches. Whichever method used, each has its merits
and limitations.

In developing any irrigation management strategy, two questions are
common: "When do I irrigate?" and "How much do I apply?" This bulletin
deals with the WHEN.

One method that is commonly used to determine when to irrigate is to
follow soil moisture depletion. As a plant grows, it uses up the water
within the soil profile of its rootzone. As the water is being used
by the plants, the moisture in the soil eventually reaches a level at
which an irrigation is required or else the plant will experience stress.
If water is not applied, the plant will continue to use what little
water is left until it final uses all of the available water in the
soil and dies.

When the soil profile is full of water, reaching what is called field
capacity (FC), the profile is said to be at 100% moisture content or
at about 0.1 bars of tension. Tension is a measurement of how tightly
the soil particles hold onto water molecules in the soil. The tighter
the hold, the higher the tension. At FC, with a tension of only 0.1
bars, the water is not being held very tightly and it is easy for plants
to extract water from the soil. As the water is used up by the plants,
the tension in the soil increases. Figure 1 shows three typical curves
for sand, clay and loam soils. As Fig. 1 shows, the plants will use
the water in the soil until the moisture level goes to thepermanent
wilting point (PWP). Once the soil dries down to the PWP, plants can
no longer extract water from the soil and the plants die. Although there
is still some moisture in the soil below the PWP, this water is held
so tightly by the soil particles that it cannot be extracted by the
plant roots. The PWP occurs at different moisture levels depending on
the plant and soil type. Some plants, which are adapted to arid conditions,
can survive with very little moisture in the soil. With most agronomic
crops, PWP occurs when the tension in the soil is at 15 bars. This means
that the soil is holding on very tightly to the water in its pores.
In order for plants to use this water, they must create a suction greater
than 15 bars. For most of the commercial crops grown, this is not possible.
At 15 bars, most plants begin to die. The difference between field capacity
and PWP is called the plant available water (PAW).

Figure 1.

A diagram of typical tension and water amount
for sand,clayand loam.(Taken from the National Engineering
Handbook, 210-VI).

rrigation targets are usually set as a percent depletion of the PAW.
This depletion, level is referred to as Management Allowable Depletion
(MAD). The bulk of irrigation research recommends irrigating row crops
such as grain or cotton when the MAD approaches 50%. For vegetable crops,
the MAD is usually set at 40% or less, because they are more sensitive
to water stress. These deficit amounts assure that water stress will
not be so severe as to cause any negative effects, and yet will allow
a little "breathing room," in case of a delayed irrigation. Careful
monitoring of the PAW needs to be done throughout the season so that
the appropriate point of irrigation can be anticipated. The following
approaches can be used to determine soil moisture content.

Determining soil moisture by feeling the soil has been
used for many years, by researchers and growers alike. By squeezing
the soil between the thumb and forefinger, or squeezing the soil in
the palm of a hand, a fairly accurate estimate of soil moisture can
be determined. It takes a bit of time and some experience, but it is
a proven method. Table 1 gives a description of "how the soil should
feel" at certain soil moisture levels. In this table, soil moisture
information is given using inches per foot (in./ft). This term (in./ft)
refers to how many inches of water are available in a foot of soil.
For example, looking at a sand (Table 1, column 1) we can see that the
wilting point is about 1.0 in./ft. This implies that a sand holds one
inch of water per foot of soil. As the soil dries, it becomes harder
to make a soil ball, and soon the soil is crumbling in your fingers.
Irrigation should occur somewhere in the shaded area, earlier for crops
sensitive to water stress.

Let's look at a clay loam. At a 0.4 in./ft deficit, a
ribbon can be easily made when the soil is squeezed between the thumb
and forefinger. Since the wilting point occurs at about 1.8 in./ft.,
a 0.4 deficit would equate to:

(0.4/1.8)*100 = 22%

a 22% deficit. A sandy loam soil makes a good ball at
0.6 in./ft deficit (about 40% deficit), but will not make a ball at
all and only sticks together at 1.0 in./ft (about 66% deficit). Once
you become familiar with the feel of the soil, it will become easier
to estimate soil moisture content. However, it is often difficult to
become familiar with the feel of the soil and this method requires much
experience.

The neutron probe has been used extensively in research
situations to determine soil moisture. A neutron probe or neutron moisture
gauge contains a radioactive source that sends out fast neutrons. These
fast neutrons are about the same size as a hydrogen atom, a critical
component of water. When the fast neutrons hit a hydrogen atom, they
slow down. A detector within the probe measures the rate of fast neutrons
leaving and slow neutrons being bounced back. This ratio can be used
to estimate soil moisture content. However, because every soil has some
background hydrogen sources that are not related to water, calibration
is important for every soil. To measure soil moisture with a neutron
probe, an access tube is installed into the ground. Then, the probe
(which contains the radioactive source and the detector) is lowered
to the desired depth (Fig. 2). Probes are quite expensive (approximately
$4500), and require an operating license because they contain radioactive
material.

Another method that has been used for several years to
determine soil moisture content is electrical resistance. Devices such
as gypsum blocks and Watermark sensors are examples of devices that
use electrical resistance to measure soil moisture. The principle behind
these devices is that the moisture content can be determined by the
resistance between two electrodes embedded in the soil. The more water
in the soil, the lower the resistance. In the early stages of development,
it was discovered that a salt bridge can form between the two electrodes,
giving false readings. Today, electrodes are embedded in better, more
stable material. The practical use of these devices is limited and they
operate best in the high range of soil moisture. To measure soil moisture,
the blocks are buried in the ground at the desired depth, with wire
leads that reach to the soil surface. A meter is then connected to the
wire leads and a reading is taken (Fig. 3). Retrieval of these instruments
is difficult in clay soils, but they are relatively inexpensive (approximately
$15 ea.).

Table 1. Description of the
soil texture parameters used to determine soil moisture using the feel
method.

Soil Texture Classification

Moisture
Deficiency

Inches/ft

Coarse

(Loamy
Sand)

Light

(Sandy
Loam)

Medium

(Loam)

Fine

(Clay
Loam)

Moisture

Deficiency

Inches/ft

(Field
Capacity)

(Field
Capacity)

(Field
Capacity)

(Field
Capacity)

0.0

Leaves
a wet outline on hand when squeezed

Leaves
a wet outline on hand when squeezed; makes a short ribbon

Leaves
a wet outline on hand when squeezed; will ribbon out about 1
inch

Leaves
a wet outline on hand when squeezed; will ribbon out about 2
inches

0.0

0.2

Appears
moist

Makes
a hard ball

0.2

0.4

Makes a weak ball

Forms
a plastic ball,

Slicks
when rubbed

Will
slick and ribbon easily

0.4

0.6

Sticks
together slightly

Makes
a good ball.

Makes
a thick ribbon

Slicks
when rubbed

0.6

0.8

Very
dry; loose, flows through fingers

Makes
a weak ball

Forms
a hard ball

Makes
a good ball

1.0

Wilting
point

Sticks
together

Forms
a good ball

Will
ball but won’t

flatten rather than crumble

1.2

Forms
a weak ball

1.2

1.4

Wilting
Point

Clods
crumble

1.4

1.6

1.6

1.8

A
“Ball” is formed by squeezing a handful of soil firmly

Wilting
Point

1.8

2.0

A
“Ribbon” is formed between thumb and forefinger

2.0

2.2

Wilting Point

2.2

2.4

2.4

Figure 3.

Diagram of resistance blocks. Here, three
blocks are anchored by a stake in the field

As previously mentioned, as soil dries out, the soil particles
retain the water with greater force. Tensiometers measure how tightly
the soil water is being held. Most tensiometers have a porous or ceramic
tip connected to a water column. The tensiometers are installed to the
desired depth of measurement (Fig. 4). As the soil dries, it begins
to pull the water out of the water column through the ceramic cup, causing
a suction on the water column. This force is then measured with a suction
gauge. Some newer models have replaced the suction gauge for an electronic
transducer. These electronic devices are usually more sensitive than
the gauges. Tensiometers work well in soils with high soil- water content,
but tend to lose good soil contact when the soil becomes too dry. Like
the resistance blocks, they are difficult to remove from clayey soils.
Costs range from $30 for small tensiometers with gauges to $2000 for
the electronic meters (reads multiple sites).

Figure 4.

Diagram of a tensiometer. In
some cases, the gauge is replaced with a connection for a transducer
that measures suction.

New devices and methods are becoming available to growers
every year. Two new techniques for soil moisture determination are instruments
using Time- Domain Reflectometry (TDR probes) and Capacitance (C-Probes,
Frequency-Domain Reflectometers).

TDR instruments work on the principle that the presence
of water in the soil affects the speed of an electromagnetic wave (slows
it down). The TDR sends an electromagnetic wave through a guide (usually
a pair of parallel metal spikes) placed into the ground at the desired
depth. It then measures the time it takes the wave to travel down the
guide and bounce back (reflect back) up the guide. The time is recorded
and converted to soil moisture. The wetter the soil, the longer it takes
for the electromagnetic wave to travel down the guide and reflect back.

C-Probes and Frequency Domain Reflectometers (FDR) use
an AC oscillator to form a "tuned" circuit with the soil. After inserting
probes that are either parallel spikes or metal rings into the soil,
a tuned circuit frequency is established. This frequency changes depending
on the soil moisture. Most models use an access tube installed in the
ground (like the neutron probe).

TDR, FDR and C-Probes have all worked well, but have their
limitations. They read only a small volume of soil surrounding the guides
or probes. FDR and C-Probes are also sensitive to air gaps between the
access tube and the soil. Many of these newer instruments require professional
installation to operate properly. In soils where caliche and other hard
pan layers exist, installing these probes may be difficult. This type
of problem is compounded when the soil is dry. Cost for these probes
range from $5000-$10,000.

Also useful in determining WHEN to irrigate are plant
indicators. Plant indicators enable the grower to use the plant directly
for clues as to when to irrigate, not an indirect parameter such as
soil or evaporative demand. Observing a plant characteristic can give
you a good idea of the status of the field's moisture content.

INFRARED/CANOPY TEMPERATURE

An infrared (IR) thermometer measures the thermal temperature
of the plant leaves or a crop canopy. Similar to humans perspiring to
keep cool, plants transpire through little openings called stomata.
Once plants go into water stress, they begin to close their stomata
and cease to transpire, causing the plant to "heat up" and the canopy
temperature to rise. Infrared readings can detect this increase in plant
temperature.

When using this method, baseline temperatures need to
be taken prior to measurements. The baseline temperature should be taken
in a well-watered field, free of water stress. On days, when the air
temperature is very high, some plants will stop transpiring for a brief
period. When plants cannot keep up with the environmental demands, they
close their stomata. If infrared readings are being taken at that time,
they may read that there is a water stress when, in fact, it is just
a normal shutdown period. Compare field readings with your well-watered
readings to make your decision. IR also requires taking temperature
readings on clear days at solar noon. This normally occurs between noon
and 2:00 p.m. This is to assure that the measurement you are taking
is at maximum solar intensity. During the monsoon season, this may be
difficult to achieve due to more cloud cover. Early in the season, IR
readings will often measure soil temperature when canopy cover is sparse.
These readings usually result in higher temperature readings since the
soil tends to heat up quickly. Figure 5 is a diagram of a hand-held
IR gun.

The use of computer programs to help schedule irrigation
was introduced in the 1970's. However, only recently with the introduction
of fast, personal computers have they begun to gain wider acceptance.
Several methods can be used to determine crop water use and help growers
schedule irrigation. The most common is to use an equation to calculate
the water use or evapotranspiration (ET) for a reference crop and relate
that to other crops. ET refers to water loss from soil evaporation and
plant transpiration. In the beginning of a crop's growing season, the
plants are small and most of the water loss is through soil evaporation.
As the plants grow and a canopy develops, the soil becomes shaded and
most of the water loss is through plant transpiration.

Reference equations include alfalfa-based equations (ETr)
and grass-based equations (ETo). There are several equations, each with
its own advantages and disadvantages. In Arizona, the Modified-Penman
equation is widely used. This equation uses weather data to predict
the water use of grass. Other equations used with some success are the
Blaney-Criddle, Jensen-Haise, and Hargreaves.

In addition to using equations to calculate a reference
ET, evaporation pans are used to determine a reference ET and then this
is related to the crop ET. Also, there are energy equations and several
other approaches to determining reference ET. Table 2 gives a list of
popular methods.

As previously stated, in Arizona, the Modified-Penman
equation has been used for several years with success. This equation
will be used in the following example of crop ET determination.

Figure 6 shows a graph of the calculated reference ET
(ETo) using the Modified-Penman equation for dry onions grown in Central
Arizona in 1996. Figure 6 also shows the measured crop water use for
the crop (evapotranspiration of the crop - ETc). Using the following
equation:

ETc = ETo * Kc

the crop coefficient (Kc) can be calculated. Using several
years of weather data and crop water use data, crop coefficients can
be determined and a specific crop curve can be developed (Fig. 7). Using
thermal time (Heat Units), these crop curves can be used in areas where
the daily temperatures differ.

Figure 7

Crop coefficient curve for dry onions developed
from ETo and ETc data from Fig.6 and two other years of data
from Maricopa AZ.

Equally important as the crop curve in irrigation scheduling
are the soil water parameters. The PAW of the soil must be known as
well as the FC.

In its simplest form, irrigation scheduling is a checkbook
balance system. For most crops in Arizona, the soil is at 100% moisture,
or very near, at planting time or just after planting. Then, using ETo
equations with crop coefficients, the daily crop water use can be determined.
This is subtracted from the total water in the soil and then a new soil
water content is determined. This continues until the amount of depletion
of PAW in the soil reaches a predetermined setting (the MAD). For many
crops, the MAD is set to 40-50% in the rootzone of the crop. Some crops,
such as vegetable crops, are more sensitive to large fluctuations of
soil moisture and the MAD are set to lower levels.

The most common irrigation scheduling methods used by
growers are: scheduling according to the calendar (number of days since
the last irrigation), looking at the crop for color change, or digging
in the field and feeling the soil to estimate soil moisture. Calendar
scheduling does not take into account weather extremes, which may cause
problems from year to year. Just looking at the crop requires experience
and a good eye - some growers have it, some do not. Even when you have
a good eye, quite often a yield loss has already occurred by the time
the plant visually shows signs of stress. Feeling the soil can give
good estimates, but is often too time consuming for many growers. Also,
when using this technique, one needs to take into account the soil profile
of the active rootzone. Estimating rootzone depth can often be difficult.

In this paper, we discussed some of the options available
to growers in helping them determine WHEN to irrigate. Whatever method
is decided upon, choosing a definite approach is always wise. Guessing
can lead to unnecessary frustration, yield loss or excess water costs
by the end of the season. Take your time and do some investigation before
you invest in any new soil moisture measuring system. An excellent place
for information is on the Internet. A site called http://www.sowacs.com
contains information on many of the instruments described in this publication.
It also provides links to companies and research reports.

The author would like to thank Jennifer Jones, Program
Coordinator at the Maricopa Agricultural Center for her editorial assistance
and the development of the graphics for this publication.

The University of Arizona is an Equal Opportunity/Affirmative
Action Employer. Any products, services, or organizations that are mentioned,
shown, or indirectly implied in this publication do not imply endorsement
by the University of Arizona.
Document locatedhttp://ag.arizona.edu/pubs/water/az1220/
Published January 2001Return to College publication list